1. Field of the Invention
The present invention relates to an equalization processing method and an equalization processing apparatus for periodical fluctuation of transmission line characteristics.
More particularly, an equalization characteristic changing method and the apparatus for detecting fluctuation of transmission line characteristics and changing the equalization characteristic according to the fluctuation of transmission line characteristics, wherein the fluctuation of transmission line characteristics periodically occurs, in a modem and the like for a power line communication, due to on-off of internal elements of a household electrical appliance which is connected to a power line such as a switching power source and the like.
2. Description of the Related Art
FIG. 1 shows a configuration example of a power line communication system. As shown in the figure, a high voltage power line 8-2 of 6.6 kV is provided between a distribution substation 8-1 and a pole transformer 8-3, and a low voltage distribution line 8-4 of 100V/200V and a service line 8-5 are provided between the pole transformer 8-3 and a house 8-6.
In the power line communication system, an optical fiber is provided in parallel with the high voltage distribution line 8-2 between an access node 8-11 in the distribution substation 8-1 and a modem in the pole transformer 8-3 so that an optical signal is transmitted. A communication signal is transmitted between the pole transformer 8-3 and a modem plugged in a wall outlet in the house 8-6 via the low voltage distribution line 8-4 of 100V/200V, the service line 8-5 and an interior distribution line 8-7.
In such a power line communication system, the low voltage distribution line 8-4 can be regarded as inductor for the modem in the pole transformer 8-3. In addition, the service line 8-5 which is connected to the low voltage distribution line 8-4 and the interior distribution line 8-7 can be regarded as a condenser. In addition, household electrical appliances show large capacitive load since condensers for preventing noise are connected between AC 100V lines.
As a result, the low voltage distribution line 8-4 is regarded as a low-pass filter (LPF) by the pole transformer 8-3 so that there is a case where a receive signal of the modem connected to the interior distribution line 8-7 is buried in noise since high frequency component is largely attenuated. On the other hand, low frequency component is not much attenuated comparing with the high frequency component. However, communication quality of the low frequency component degrades since random noise (colored noise) is released from switching power sources or inverter circuits of the household electrical appliances so that the low frequency component is also buried in large noise. Therefore, taking measures against the noise is necessary for realizing high speed data communication by the power line carrier.
For taking measures against degradation of the receive signal, frequency modulation (FM), frequency-shift keying (FSK), phase-shift keying (PSK) and the like which are said to be strong for noise are adopted conventionally as modulation method of the modem for power line communication. However, since noise level of power line is high, thus, applications which are realized by these modulation methods are limited to low speed applications under 1200 bps.
In addition, spread spectrum technology has been used for the power line communication. However, transmission capacity decreases remarkably when S/N value becomes minus under white noise environment. Thus, data can be sent at 100 kbps at the maximum, and communication is interrupted at worst. In addition, an attempt has been made to apply a technology to avoid carrier band in which noise is high, by using OFDM (Orthogonal Frequency Division Multiplexing) which uses multi-carrier modulation method.
However, the switching power sources in the household electrical appliance and the inverter circuits tend to increase. Conventional technology can be used for low speed communication even when noise and fluctuation of transmission line characteristics due to switching of the power sources and the inverter circuits occurs. However, it is not possible to realize the power line communication at high speed of several Mbps.
The switching power sources and the inverter circuits in the household electrical appliance are more and more increasingly used. In addition, condensers are connected to 100V power source lines for preventing noise so that capacitive load is increasing. Under these circumstances, it is not enough to avoid noise for communication in the way as conventional technology. On the other hand, it is more effective to realize high speed communication by actively coping with the occurrence of noise and fluctuation of transmission line characteristics so as to increasing receiving accuracy of a signal.